An internal combustion engine capable of using fuels having different properties is mounted in a so-called FFV (flexible-fuel vehicle). Ethanol-blended gasoline may be typically used with such an FFV internal combustion engine. When the ethanol-blended gasoline is used as a fuel for an internal combustion engine, it is necessary to adjust an air-fuel ratio in accordance with the concentration of ethanol in the fuel because the ethanol greatly differs from gasoline in calorific value per unit volume. Therefore, an internal combustion engine using the ethanol-blended gasoline has an ethanol concentration sensor, which is a fuel property sensor for determining the properties of an employed fuel, or more specifically, the ethanol concentration in the employed fuel. It is preferred that a capacitance sensor, an optical transmission sensor, or an optical refractive-index sensor be used as the ethanol concentration sensor.
The ethanol concentration measured by the ethanol concentration sensor is used as a parameter for internal combustion engine air-fuel ratio control. This makes it possible not only to obtain a desired torque but also to ensure satisfactory emission performance without regard to the ethanol concentration in the employed fuel.
As described above, the fuel property sensor in the FFV internal combustion engine plays an important role to ensure the expected performance of the internal combustion engine. However, there is no guarantee that the fuel property sensor will function normally at all times, as is the case with the other sensors. Wiring disconnection, short-circuiting, sensor element deterioration, or other abnormality may occur in the fuel property sensor. If, in such an instance, the internal combustion engine is controlled by using an output value of the fuel property sensor, the internal combustion engine fails to operate appropriately in accordance with the properties of the employed fuel, thereby degrading the performance characteristics of the internal combustion engine such as emission performance and fuel efficiency.
It is therefore demanded that an abnormality in the fuel property sensor be accurately detected to immediately repair or replace the fuel property sensor or take other appropriate remedial action. In view of the above circumstances, a technology disclosed in JP-A-2010-038052 (hereinafter referred to as Patent Document 1) presets an upper-limit threshold value and a lower-limit threshold value for the output value of an ethanol concentration sensor. When the output value is outside the range between the upper- and lower-limit threshold values, this technology concludes that the ethanol concentration sensor is abnormal. As the output value of the ethanol concentration sensor varies with fuel temperature even when the ethanol concentration remains unchanged, this technology can change the upper- and lower-limit threshold values in accordance with the fuel temperature measured by a fuel temperature sensor.
However, the technology disclosed in Patent Document 1 does not accurately detect an abnormality in the ethanol concentration sensor at all times. A phenomenon called “stuck” is an abnormality that is likely to occur particularly in the ethanol concentration sensor and will greatly affect the control of the internal combustion engine. In this phenomenon, the output value of the ethanol concentration sensor is stuck at a fixed value. However, this phenomenon may also occur when the output value of the ethanol concentration sensor is between the upper- and lower-limit threshold values. Therefore, the technology disclosed in Patent Document 1 may fail to detect this phenomenon as an abnormality.
A method of detecting a stuck of a capacitance temperature is well-known, as described in JP-A-2000-303898 (hereinafter referred to as Patent Document 2). The method described in Patent Document 2 calculates the difference between a maximum water temperature and a minimum water temperature, which are measured by the temperature sensor after startup of an internal combustion engine. When the calculated difference is small, this method concludes that the sensor is stuck. However, it is difficult to apply this method to the detection of a stuck ethanol concentration sensor. The reason is that, unlike fuel temperature, the ethanol concentration in fuel cannot be changed without refueling.
When the output characteristics of the ethanol concentration sensor relative to the fuel temperature, which are described in Patent Document 1, are taken into account, whether or not the ethanol concentration sensor is stuck can be determined by checking whether the output value of the ethanol concentration sensor varies with the fuel temperature. However, if the ethanol concentration in the employed fuel is 0%, the output value of the ethanol concentration sensor remains substantially unchanged even when the fuel temperature varies. Therefore, the above-described method cannot determine whether the ethanol concentration in the employed fuel is 0% or the sensor is stuck.
Another method of detecting an abnormality in the fuel property sensor is described in JP-A-2008-014741 (hereinafter referred to as Patent Document 3). The abnormality detection method described in Patent Document 3 presumes that the inlet of a fuel tank is provided with a measurement chamber, which includes a fuel property sensor. It is also presumed that the level of a signal output from the fuel property sensor varies depending on whether fuel exists in a measurement space within the measurement chamber. When the employed configuration is as described above, no fuel stays in the measurement space under normal conditions. However, when the fuel is supplied to the fuel tank, the fuel temporarily stays in the measurement space. Thus, the fuel in the measurement space changes the signal level output from the fuel property sensor. Therefore, if the fuel property sensor does not output an appropriate signal during refueling, it can be concluded that the fuel property sensor is abnormal.
However, the technology described in Patent Document 3 cannot determine the properties of an employed fuel with adequate accuracy. The information about fuel properties necessary for internal combustion engine control is the information about the properties of the fuel supplied from the fuel tank to the internal combustion engine, or more specifically, the information about the properties of the fuel injected from an injector. The description set forth in Patent Document 3, however, states that fuel properties determined by the fuel property sensor are the properties of the fuel supplied to the fuel tank and not the properties of the fuel injected from the injector. In the FFV internal combustion engine, which can use fuels having different properties, the properties of the fuel in the fuel tank do not always agree with those of a newly supplied fuel. Therefore, it is highly probable that the fuel properties determined by the fuel property sensor based on the technology described in Patent Document 3 may differ from the fuel properties of the fuel injected from the injector. This makes it difficult to provide appropriate air-fuel ratio control in accordance with the properties of an employed fuel.
Further, the technology described in Patent Document 3 cannot detect an abnormality in the fuel property sensor with adequate accuracy, or more specifically, cannot determine with adequate accuracy whether the fuel property sensor is stuck. If, for instance, the output value of the fuel property sensor is stuck at an output level at which no fuel exists in the measurement space, detecting the output level of the fuel property sensor during refueling makes it possible to determine whether the fuel property sensor is stuck. However, if the output value of the fuel property sensor is stuck at an output level at which the fuel exists in the measurement space, the output level remains unchanged during refueling. Therefore, the fuel property sensor is found to be operating normally. In other words, the technology described in Patent Document 3 cannot detect a stuck sensor in such a situation.
As described above, the previously proposed technologies for fuel property sensor abnormality detection do not detect an abnormality in a fuel property sensor with adequate accuracy, or more particularly, do not detect a stuck fuel property sensor with adequate accuracy.